Field of the Invention
This invention relates to a power conversion device that performs power conversion by repeating an ON operation and an OFF operation of a semiconductor switch.
Description of the Related Art
In this type of power conversion device, since switching control is performed at a high-frequency switching frequency which is generally assumed to be 20 kHz or higher, high switching noise caused by an ON operation or an OFF operation of a switching element is generated. As a result, as a noise generation source, it may cause harm such as a malfunction or a shutdown of other electronic devices. In actuality, for such noise, since there is a need to ensure compatibility among standards of various countries, International Standard IEC (International Electrotechnical Commission) establishes and publishes EMC (Electromagnetic Compatibility) standards of electronic devices and automobile devices in various fields. In order to suppress such switching noise, generally, providing a noise countermeasures part may be possible, but this inevitably results in an increase in cost and an increase in the size of a device.
Thus, in the past, a method described in JP-A-2011-193593 (Patent Document 1), for example, has been proposed. That is, in accordance with the method proposed in Patent Document 1, common-mode noise is effectively reduced by inserting reactors in both of a line leading out of a power supply and a line returning to the power supply, and, as a result of a core being shared by the reactors which are inserted into both lines, since magnetic fluxes generated in both reactors are added to each other, it is possible to increase the inductance of the reactors and make the reactors smaller.
[Patent Document] JP-A-2011-193593
However, in accordance with the method proposed in Patent Document 1 described above, the capacity of a line-to-ground bypass capacitor connected between the line and the ground of a filter circuit and the common-mode inductance of the reactors which are inserted into both lines produce resonance, and the common-mode noise characteristics are deteriorated in a resonance frequency band. If the combined capacity of the line-to-ground bypass capacitor is assumed to be C1 and the common-mode inductance of the reactors which are inserted into both lines is assumed to be L1, a resonance frequency fr is expressed by Equation (1).fr=½π√(L1C1)  (1)
As the combined capacity of a line-to-ground bypass capacitor, the maximum capacity is determined from an admissible leakage current value and is generally about several dozen nF. Moreover, the inductance value of a reactor for energy accumulation, for example, is determined from an admissible ripple current and is a few hundred μH in a several-kw class power conversion device. Based on these facts, the resonance frequency by the capacity of the line-to-ground bypass capacitor and the common-mode inductance of the reactors which are inserted into both lines is about a few hundred kHz.
Here, in general, the Fourier spectral envelope of the voltage across a switching element such as an IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) at the time of switching operation is expressed as that depicted in FIG. 5. T of FIG. 5 denotes a period, f0 denotes a fundamental component of a switching frequency, τ is a pulse width, tr is a rise time, and tf is a fall time. As depicted in FIG. 5, the frequency of a harmonic component of f0 which is higher than or equal to f1 expressed by Equation (2) is the locus of the maximum amplitude, that is, the Fourier envelope attenuates at −20 dB/decade.f1=1/πτ  (2)
As expressed by Equation (3), f2 depicted in FIG. 5 depends on the rise time tr, and, when the frequency becomes higher than or equal to f2, the Fourier envelope becomes −40 dB/decade and the attenuation characteristics become further greater.f2=1/πtr  (3)
The above description reveals that the attenuation of a harmonic component of a fundamental of a switching frequency becomes greater at a frequency higher than or equal to f2. The rise/fall time of a switching operation of a recent IGBT, MOSFET, or the like is about a few hundred ns and f2 at this time is a few MHz.
That is, in the technology proposed in Patent Document 1, since no consideration is given to the resonance produced by the capacity of the line-to-ground bypass capacitor and the reactors which are inserted into both lines and the resonance frequency fr thereof generally becomes a few hundred kHz and fr<f2, the common-mode noise characteristics are deteriorated in a band in which the attenuation of a harmonic component of a fundamental of a switching frequency is small. As a result, it is necessary to provide a noise countermeasures part such as a common-mode choke separately or improve the common-mode noise characteristics, which results in an increase in the size of a noise filter and an increase in cost.